Under circumstances where the price of energy is rising worldwide, development projects of a large-scale gas field of seabed far from land are currently beginning in earnest. A LNG-FPSO used for this has functions of liquefying gas, which is produced from a gas field of seabed, on the ocean to produce LNG, storing the LNG in a tank, and loading it onto a LNG carrier. (Note that a LNG-FSRU has a function of gasifying LNG received from a LNG carrier to deliver the LNG to the land.)
The LNG-FPSO includes a tank for storing a large amount of the produced LNG, and as its tank structure, LNG tank technology, which has been fostered in the construction of a conventional LNG carrier, is expected to be adopted. However, since how the LNG storage tank is used is different between the LNG-FPSO and the LNG carrier, care is needed. In the case of the LNG carrier, a phenomenon that a liquid cargo in the tank violently sloshes (sloshing phenomenon) is unlikely to occur even at the time of heavy weather because the LNG storage tank is used either in full load condition or in unload condition and is never in half load condition. Only at the time of a cargo handling work, a liquid level in the tank greatly changes, but since the cargo handling work has been usually performed in a port where waves and winds are quiet, it has been possible to almost disregard the sloshing.
On the other hand, in the LNG-FPSO, the sloshing phenomenon is thought to be likely to occur because it is constantly moored on the ocean where a weather condition is severe and a liquid level in its LNG storage tank changes from time to time according to a production amount of LNG and a loading amount to the LNG carrier, and half load condition daily occurs. Another important thing about the LNG-FPSO is that loading the liquid cargo to the LNG carrier with the use of a loading arm by STS (ship to ship), in particular, while the LNG carrier is alongside the LNG-FPSO (side by side) is now under consideration. Considering that the cargo handling for a conventional LNG carrier has been performed while the LNG carrier is is moored at a berth provided in a safe port, it is thought that the aforesaid STS cargo handling on the ocean has a high risk, and a collision accident occurs between the LNG-FPSO and the LNG carrier trying to approach it to damage the hull, or an accident such as the damage of the hull could occur due to leakage of the liquid cargo from the loading arm. Therefore, in designing the tank of the LNG-FPSO, it is necessary to take such risks into full consideration.
Further, it is also necessary to take it into consideration that a future LNG carrier is loaded with LNG from a LNG-FPSO on the ocean where the weather condition is severe, and in particular, a measure against the sloshing during the loading is required.
LNG storage tanks conventionally used in LNG carriers include a self-supporting spherical tank (MOSS type tank), a self-supporting prismatic tank (SPB type), and a membrane tank, and it is expected that one of these three tank types will be adopted also in the LNG-FPSO. Firstly, regarding the self-supporting spherical tank, it is a self-supporting tank made of an aluminum alloy and is supported in a hold formed by a double hull, via a skirt extending from its equatorial portion. A heat insulation layer is applied on an outer surface of the tank (external thermal insulation). Due to its spherical shape, the self-supporting spherical tank has a disadvantage of low volumetric efficiency because it is not well fitted in the hold. In the tank of this type, owing to its external thermal insulation, the heat insulation layer does not suffer damage even when a cargo sloshes at the time of heavy weather.
In the self-supporting prismatic tank, a main body is a prismatic tank made of an aluminum alloy and strength members reinforcing the tank are provided on an inner side of the tank, and a heat insulation layer is provided on an outer surface of the tank. This type requires void space between the prismatic tank and an inner hull of the ship, which accordingly reduces volumetric efficiency of the tank. On the other hand, since the strength is members can be provided inside the tank, sloshing of a liquid cargo does not easily occur at the time of heavy weather, and even if the sloshing occurs, the heat insulation layer provided on the outer surface of the tank is not damaged.
Regarding the membrane type, on an inner surface of a hold fabricated with a double hull structure, thin sheets (membranes) made of nickel steel or stainless steel are affixed, with a heat insulation layer therebetween, to form a LNG tank. This type is excellent in volumetric efficiency because almost all the volume of the hold can be used as a tank volume. On the other hand, it has a disadvantage that the membranes and the heat insulation layer are likely to be damaged by the sloshing of a liquid cargo. It also has a problem that a thermal insulation work, in particular, the welding of the membranes is complicated and it requires a long period for the construction.